Physiology of the Menstrual Cycle

menopausa donne consigli
  1. What is the menstrual cycle?
  2. Which are the phases of the menstrual cycle?
  3. Which are the principal hormones involved?
  4. What happens during the menstrual cycle?

What is the menstrual cycle?

The menstrual cycle is a sequence of physiological and periodical changes of the feminine reproductive system. It is regulated by hormones produced by the ovaries and the hypophysis. All women have their first menstrual cycle during puberty and will cyclically have it until menopause, with interruptions only during pregnancy. The ovarian cycle, and likewise the menstrual cycle, lasts on average 28 days and consists of three phases which follow one another and that are strictly correlated to the hormones that are secreted during the cycle.

Which are the phases of the menstrual cycle?

  • Menstrual phase: during which the endometrium tends to flake off and is expelled through the vagina with menstruation
  • Proliferative phase: during which the endometrium is restored
  • Secretory phase: during which the endometrium becomes thicker to accommodate the fertilized egg

Which are the principal hormones involved?

Sometimes the menstrual cycle causes abdominal pain during the pre-menstrual phase with a series of disorders that affect women during the days immediately before menstruation. Menstrual cycles, also known as periods, last 25-35 days, with an average of 28 days for women aged in their 20s and 26 days for women in their 40s. In a normal menstrual cycle, cyclical changes of the following four principal hormones take place:

  • Luteinizing hormone (LH)
  • Follicle-stimulating hormone (FSH)
  • Oestrogen
  • Progesterone

The process involving changes in these hormones is usually associated to a change in body temperature. Through the central nervous system, the hypothalamus releases the gonadotrophin-releasing hormone (GnRH), which regulates the release of LH and FSH, which then subsequently regulate the secretion of oestrogen. After this, the levels of FSH and oestrogens increase, ovarian folliclesdevelop and mature. About seven days after ovulation, if no fertilization has occurred, the production of hormones starts to slow down. It is during the last days of the cycle, with the decline in hormonal levels and the first days of menstruation, during which such hormonal levels are low, that women are affected by the most annoying disorders. Cycles are counted from the first day of the menstrual flow when the levels of all four hormones are low.

What happens during the menstrual cycle?

Menstrual bleeding is the external symptom of the above-mentioned cycle in women, which occurs at the end of the luteal and at the beginning of the follicular phases of the ovarian cycle. In 80% of women during ovulation, menstrual bleeding occurs for 3-6 days with heavier flow on the second day with a loss of about 33 ml of blood (10-84). In women aged 35-50, a reduction of bleeding can be observed, while older women usually lose 6 ml more than younger ones. The menstrual cycle is closely linked and controlled by endocrine, autocrine and paracrine factors that regulate follicular development, ovulation and endometrial remodelling.